This invention relates to laser devices, and more particularly to mechanism for stabilizing the oscillation wavelength of the laser beams of laser devices.
Laser devices such as excimer lasers and some of the solid state lasers including semiconductor lasers have relatively wide oscillation bandwidths. Thus, when laser beams of such laser devices are utilized for fine machining, etc., the chromatic aberrations generated by converging lenses cause problems. It has therefore been proposed to insert etalons within the laser resonator of the laser device so as to narrow the bandwidth of the laser beam and obtain a substantially monochromatic laser beam.
FIG. 1 shows such a laser device which is disclosed, for example, in Japanese Laid-Open Patent Application (Kohai) No. 1-205488. A laser resonator 1 consists of a laser medium 2, a totally reflective mirror 3, and a partially reflective mirror 4. Within the laser resonator 1 there are disposed a rough adjustment etalon 5 which roughly selects and narrows the bandwidth of the laser beam, and a fine adjustment etalon 6 which further narrows and determines the wavelength of the laser beam. As shown in FIG. 2, each of these etalons comprises a pair of parallel transparent plates 5a opposing each other across a gap d. A reflective coating 5b is formed on the opposing surface of each of the plates 5a. The central transmission wavelength of the etalons can be adjusted by changing the gap d between the plates 5a or the angle of the etalons with respect to the laser beam. The laser beam 7 is outputted from the laser resonator 1 after being narrowed in bandwidth via the rough adjustment etalon 5 and fine adjustment etalon 6. A first interference fringe detector 9 detects the interference fringes formed by the laser beam 7 reflected by the partially reflective mirror 8. As shown in FIG. 3, the first interference fringe detector 9 comprises: an integrator 10 for weakening and diffusing the light for forming the interference fringes, an etalon 11, a lens 12, an imaging element 13 for detecting the positions where the light concentrates, and an image processing unit 14. A first etalon control mechanism 15 adjusts the transmission wavelength of the fine adjustment etalon 6 by changing the gap length d or the angle of the fine adjustment etalon 6 so as to adjust the interference fringes to the predetermined interference fringe pattern of a laser beam having a predetermined oscillation wavelength.
A light source 16 emits light the bandwidth of which is narrowed only by means of the rough adjustment etalon 5. The light pencil 18 emitted from the light source 16 is converged by a converging lens 17 and goes through the rough adjustment etalon 5 to be narrowed in its bandwidth. A second interference fringe detector 20 detects the interference fringes formed by the light pencil 18 emitted from the light source 16 after transmitting through the rough adjustment etalon 5 and reflecting at the reflection mirror 19. As shown in FIG. 3, the second interference fringe detector 20 comprises a lens 21 for forming the interference fringes, an imaging element 22 for detecting the positions where the light is concentrated, and an image processing unit 23. The interference fringes formed on the imaging element 22 within the second interference fringe detector 20 are generated by the light 18 the bandwidth of which is narrowed only via the rough adjustment etalon 5. A second etalon control mechanism 24 controls and changes the transmission wavelength of the rough adjustment etalon 5 by adjusting the gap length d or the angle of the rough adjustment etalon 5 such that the interference fringes form in the second interference fringe detector 20 are adjusted to the interference fringe pattern corresponding to a predetermined oscillation frequency of a laser beam. A selection control mechanism 25 determines whether it is necessary to control the rough adjustment etalon 5 and fine adjustment etalon 6, and when it is necessary, judges the priority of the control thereof.
The method of operation of the laser device is as follows. The light generated in the laser medium 2 bounces back and forth between the totally reflective mirror 3 and partially reflective mirror 4 and thus is amplified within the laser resonator 1. The amplified light goes out of the laser resonator 1 as the laser beam 7. Since the rough adjustment etalon 5 and fine adjustment etalon 6 are inserted within the laser resonator 1, the oscillation bandwidth is narrowed, and hence a substantially monochromatic laser beam 7 can be obtained.
The principle of bandwidth narrowing by means of the rough adjustment etalon 5 and fine adjustment etalon 6 is as follows. FIG. 4 shows the principle by which the oscillation bandwidth of laser beam is narrowed. FIG. 4(a) shows the spectral transmission characteristic of the rough adjustment etalon 5. The central transmission wavelength .lambda.m.sub.1 are given by the following equation (1) EQU .lambda.m.sub.1 =2 n.sub.1 d.sub.1 cos .theta..sub.1 /m.sub.1 ( 1)
wherein:
n.sub.1 represents the reflectivity of the material filling the space between the two mirror surfaces of the etalon;
d.sub.1 represents the distance between the two mirror surfaces of the etalon;
.theta..sub.1 represents the incident angle of the laser beam on etalon; and
m.sub.1 is an integer whose distinct values correspond to the respective transmission peaks of the etalon.
As can be clearly seen from this equation, the wavelengths at the transmission peaks can readily be adjusted at will by changing the values of n.sub.1, d.sub.1, and .theta..sub.1. On the other hand, the region between the transmission peaks are known as free spectral regions (FSR), which are given by the following equation (2): EQU FSR.sub.1 =.lambda.m.sub.1.sup.2 /2n.sub.1 d.sub.1 cos.theta..sub.1 ( 2)
Further, the half value width of the transmission peaks .DELTA..lambda..sub.1 is given by the following equation (3): EQU .DELTA..lambda..sub.1 =FSR.sub.1 /F.sub.1 ( 3)
where F.sub.1 is a value known as finesse which is determined by the performance of the etalon.
On the other hand, FIG. 4(c) shows the spectroscopic characteristic of the gain of the laser medium 2. If the etalons are not disposed within the laser resonator 1, the light is amplified in the bandwidth range where the gain is present, and hence a laser beam of wide oscillation bandwidth is generated. There is inserted, however, the rough adjustment etalon 5, and the parameters (such as d.sub.1) of the rough adjustment etalon 5 are selected such that one and only one transmission peak position .lambda.m.sub.1 of the rough adjustment etalon 5 is within the gain region of the laser medium 2. In the case shown in the figure, the peak transmission wavelength .lambda.m.sub.1 of the rough adjustment etalon 5 is at the central wavelength.lambda..sub.0 of the gain of the laser medium 2, and the adjacent transmission peaks are outside of the gain region of the laser medium 2. Thus, the attenuation due to the rough adjustment etalon 5 is small only in the neighborhood of the central wavelength.lambda..sub.0, and the light is amplified only near at .lambda..sub.0, thereby generating a laser beam narrowed in its oscillation bandwidth.
In order to limit the number of the transmission peaks present within the gain region to one, the free spectral region FSR.sub.1 must be greater than a minimum determined by the width of the gain region of the laser medium 2. On the other hand, the finesse F.sub.1, which is determined by the performance of etalon, is about 20 at most. Thus, the narrowing of bandwidth by means of rough adjustment etalon 5 alone has its limit. Thus, another etalon, fine adjustment etalon 6, is utilized. The spectroscopic transmission characteristic of the fine adjustment etalon 6 is shown in FIG. 4(b). A peak transmission wavelength .lambda.m.sub.2 thereof is turned at the central wavelength .lambda..sub.1 of the laser medium 2, and the free spectral region FSR.sub.2 thereof is selected at a value greater than .DELTA..lambda..sub.1 (FSR.sub.2 &gt;.DELTA..lambda..sub.1).
Thus, the laser bean, generated by the laser medium 2 and having the spectroscopic characteristic as shown in FIG. 4(c), is narrowed in oscillation bandwidth, as shown in FIG. 4(d), to a narrow band around the central wavelength .lambda..sub.0 at which the transmission peaks of the rough adjustment etalon 5 and fine adjustment etalon 6 overlap each other. Since, the light goes back and forth many times through the etalons, the bandwidth of the laser beam is narrowed to from one half to tenth (1/2 to 1/10) of the bandwidth as determined by the transmission characteristics of the two etalons.
Where it is desirable to further reduce the bandwidth of the laser beam, another etalon may be inserted within the laser resonator 1.
The oscillation bandwidth of the laser beam can be narrowed as described above. When, however, the laser beam goes back and forth through the etalons in oscillation, heat is generated in the etalons, and, as a result, the etalons are deformed as shown in FIG. 5. These thermal deformations of the etalons, while not so severe as to deteriorate the performance characteristics of the etalons, do change the gap length d of the etalons, and thereby shift the central transmission wavelength thereof. The circumstance is shown in FIG. 6. FIG. 6(a) shows the spectroscopic transmission characteristic of the rough adjustment etalon 5, where the solid curve represents the characteristic immediately after the start of the oscillation, and the dotted curve represents the shifted characteristic after etalon has been heated. The relation between the shift of the transmission peak .DELTA..lambda. and the variation .DELTA. d of the gap d is given by the following equation (4): EQU .DELTA..lambda.=(.lambda.m/d) .DELTA.d (4)
Incidentally, the direction of the shift of wavelength is determined by the structure of the etalon. With respect to a particular etalon, the peak transmission wavelength is shifted in a certain direction due to the thermal deformation caused by the laser beam.
Not only the peak transmission wavelength of the rough adjustment etalon 5, but also that of the fine adjustment etalon 6 is shifted as shown by the dotted curve in FIG. 6(b). The gap length of the fine adjustment etalon 6, however, is greater than that of the rough adjustment etalon 5, such that the transmission wavelength shift of the fine adjustment etalon 6 is smaller than that of the rough adjustment etalon 5. Thus, central peak transmission wavelengths .lambda.m.sub.1 and .lambda.m.sub.2 of the etalons 5 and 6 become separated from each other. The overall transmission characteristic of the two etalons 5 and 6 superposed on each other is therefore reduced, as shown in FIG. 6(c), compared with the case where the central transmission wavelengths .lambda.m.sub.1 and .lambda.m.sub.2 are equal to each other. Thus, after a long time subsequent the start of oscillation, not only the oscillation wavelength of laser beam is shifted from .lambda..sub.0 to .lambda.m.sub.2, but also the output power is decreased. Furthermore, when the wavelength shifts are large, oscillation in another adjacent mode of the fine adjustment etalon 6 may be observed (see FIG. 6(c).
Thus, control is effected to stabilize the oscillation wavelength of the laser beam as follows. Part of the laser beam 7 is guided to the first interference fringe detector 9 via the partially reflective mirror 8 and is diverged by the integrator 10 (see FIG. 3). Only the components of the light diverged by the integrator 10 having particular incident angles .theta. to the etalon 11 are transmitted therethrough to reach the lens 12. When the focal length of the lens 12 is represented by f, the light having the incident angle .theta. is concentrated at positions separated from the lens axis by a radial distance f.theta., and thereby forms a circular interference fringe. The imaging element 13 detects the positions at which the light is concentrated, and the image processing unit 14 analyses the detected result, thereby obtaining the incident angel .theta., from which the current oscillation wavelength of the laser beam can be calculated. The oscillation wavelength of the laser beam is determined solely by the transmission characteristic of the fine adjustment etalon 6. Thus, the fine adjustment etalon 6 is adjusted, via the first etalon control mechanism 15, with respect to its angle to the laser beam, or its gap length d, such that the central transmission wavelength of the fine adjustment etalon 6 is tuned to the predetermined wavelength. The oscillation of the laser beam is thus adjusted to the predetermined wavelength.
The control of the rough adjustment etalon 5, on the other hand, is effected as follows. The light emitted from the light source 16 reaches the rough adjustment etalon 5, and the components having particular incident angles are thereby selected. The light thus elected via the rough adjustment etalon 5 is transmitted through the fine adjustment etalon 6 without further selection. Then, the light is reflected by the reflection mirror 19, which has a particularly high reflectivity to the light at the wavelength of the light source 16, and thence is guided to the second interference fringe detector 20. The light is then converged by the lens 21, to form circular interference fringes generated by the selection of the light via the rough adjustment etalon 5 (see FIG. 3). The imaging element 22 detects the positions where the light is concentrated, and the image processing unit 23 analyses the detected result, thereby obtaining the central transmission wavelength of the rough adjustment etalon 5. The angle or the gap length of the rough adjustment etalon 5 is controlled by means of the second etalon control mechanism 24, so as to tune the central transmission wavelength of the rough adjustment etalon 5 to the predetermined wavelength.
The above laser device, however, has the following disadvantage.
FIG. 7 shows the relation between the reflectivity of the rough adjustment etalon 5 and the intensity of the interference fringes. When the reflectivity is small, the variation of the intensity of light is also small and the interference fringes are obscure. Thus, the detection of the interference fringes by the imaging element 22 is difficult, and hence an accurate control of the rough adjustment etalon 5 is difficult to perform.
Thus, in order to perform an accurate control of the rough adjustment etalon 5, the reflectivity of the reflective surface 5b of the rough adjustment etalon 5 must be made large enough to ensure a formation of distinct and clear interference fringes in the second interference fringe detector 20. Otherwise, erroneous control may ensue.
On the other hand, increasing the reflectivity of the etalon signifies increasing the number of reflective layers constituting the reflective surface 5b of the etalon. This makes the production of the etalon difficult. Further, when the reflectivity increases, the absorption of light also increases. This reduces the resistance of the etalon to the light.